Radio receiver for multiple signals



July l0, 1951 D. H. MITCHELL RADIO RECEIVER FOR MULTIPLE SIGNALS Filed Sept. 6, 1945 mm 893m umm nono N INVENTOR. DONALD H. MITCHELL ATTORNEYS Patented July 10, 1951 UNITED STATS@ OFFI CE! RADIQ RECEIVER'FOR MULTIPLE SIGNALS,

DonaldA H. Mitchell, Chicago, Ill.,;assignor yto` Motorola, Inc., Chicago, Ill., a corporation of; Illinois Application September 6, 1945,-Serial'N'o. 614,641

10 Claims. `(Cl. Z50-#43:)

Thissinvention relatesto aircraft radio receiv-v ersfand-,.fWhile itisloiqgeneral application, itis entertainment or. Abeacon and; marker signals for` navigationalpurposes:

Heretofore, in orderto provide v.for theproperfr navigation of aircraft-andl to provide fori-theres` ception trof ybroadcast;signals for entertainment;n n it rhaszi :beennecessary to equip an` aircraftfwitli'- ception ;ofi lmarker signals at:` .'75A mc.Y transmitte as a verticalbeam by a 4.transmitter-.;locatedxcat;.:1A

each airport lin order y'toL' identify andY locatelthe :11 air :zportzfor pilots flying byv instruments. Also, in pertain receiverspprovisions 'haveV been-.imade:

for: utilizing -the, .-audioz-,frequency portion rofzzthe receiver-:as va part .of an intercommunicating. sys` temnbutsin. such arrangements provisionsphaveu alwaysl been-.included for disabling thertransmit;

teraorthe receiver,- or bothrwhile intercommunia:

In accordance wthlth'e invention. van aircraft radio receivervcomprises* a :voice-'signal f carrier-K frequency selectingamplifyingand detecting.-frstf:`4 portion: .an .t auxiliaryA marker-signal-` carrier-free*Y .quencyr selecting; and detecting.v .second e`portion,".=

andatleastzone and preferably two or vmore in-:r` tercommunicating.'.:microphonesz:i The .receiverwY also includes an. audio-.frequency amplifier-havn ingV input, and output 1ele`ctrodes,.a circuit for apr.4v plyingctothe input electrode'sa'voice signal-rtrans-f.- lated by the rst .portion,:a circuitioif selectively? -f simultaneously: .applying Ato the.- 'input :electrodes a marker:signal'atranslatedf by the second. portion, and .a circuit'for:simultaneously applyingto the.r input 'electrodes-.a signal:.from the microphone. Thegreceiver: also: in'cludese a .signal reproducerrf coupled to theoutput electrodes of .the.amplif`1er',. whereby:y the; 'amplifierf andy reproduced- :seryefsi-i'. D multanoeusly for'rthe translation of received-.voicesignals, marker signals and intercommmication` a signalsf;A In a preferredembodiment 'of'thein- Ventiongthe system also serves zas a transmitten f and" includes Ameans i. for -Selectivelyfcoupling -1 the output electrodesA of the-.-amplier .tovazcarrier-w catingfz'andf also e have :included :complicated t :5 frequency oscillator-modulatorl vand :power am-Y 4 switohingrmechanisms forthis purpose.:l Ori-the.: pler portion, whereby.theaudio-frequencrame-v otherihand; itgisluigl'ily| desirablez'to'; providesan.. pliiier,y also servesiras :a modulationamplifier for aircraft transmitterfreceiver system ..including..; the z'microploonel While, the .system operates :as provisions for receivingbroadcast entertainment a transmitter. signals,.beacon signals-and marker signals.,as.30 Further inaccordance withthe invention: an welt-asoperating ,as .a portion of an intercomaircraftradio receiver comprises. a voice-signal:E municatingsystem, the. system being `capable-of;4 carrier-'frequency selecting, amplifyingl and .'deall of such operations simultaneously with-a min-fx, te'ctingr r.first portion, anI :auxiliary vmarker-signa-lf.v imumv. of interferenceand a vminimumgof switcl1l carrier-frequency selecting and detecting/second'E ing. 35 portion, and an audio-frequency amplifier havingw: It'is an object of the invention,l theref-ore,z1to.;: input-and outputelectrodes. Thefrecei'ver 'also provide a `nevv and improved aircraft .radio ;re;-.vv includes a circuit for applyingto the. input-elec-l ceiverlawhich.. is eieotiveto overcome onezor moreyr trodesxa :voice: signal translated .by rthe.iirst.porf of the .above-mentioned limitations andysh'orte tion,` a. -circuitl for selectively? simultaneously ap-,-- comings vof-suchxreceivers ofthe priorfaartxv lo plying tothe .input electrodes. va marker* signal-'4 Itfisjanother: object of'tlfiefinventio'nto;provide.;` translated by the .secondwpor-tion,` and asignaln anew .andimprovedaircraft radio :receiver: ini.-i reproducer coupled. to .the output:v electrodes, which :voice signals, forexample broadcast.'.sig.l whereby the amplifierffand.reproducerr servesil nals and markersignalspmay be simultaneously multaneously forthetranslation of'. received voice received Landi in whichyatrthe vsameatime, the 45 SignalS and.HIM-kielSignals:r audio-Afrequency portion ofthe receiver. serves las Further in. accordance .with the invention; an a portion i-of. an,intercommunicatingisystemf.; aircraft Vradiok .receiver comprises a voice-Signat" It4 is another object of-.the invention.topro-Y carrier-frequency selectinafamplifying and deiK videga :new and. improved transmittersreceiver fleeting ISt pOltOl, "at least One "and "p1`f1"'-y systeminoluding.areooverof.thetypedesoribedzsu ably@ vplurality 0f intemornmuncatins microinwhich the operating condition of thegsystemphones; and-'an audio-frequency 'amplier'havmaybe-.changed from receivingtotransmitting y 111gV input and Output11Ctf0d$-v The-receiver* withoutgimpairing .theffunctionof the systemfforzf: also-includesa circuit for applying 'tO'i'Ie input thev reception of` marker signalsmr inr servingqasggg electrodes a voice-signal translated-why the rstf-P an.interoommunication.system.A o5 portionga circuit forgsimultaneously applyinghftow-ithe input electrodes a signal from the microphone, and a signal reproducer coupled to the output electrodes, whereby the amplifier and reproducer serve simultaneously for the translation of voice signals and intercommunication signals.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, while its scope will be pointed out in the appended claims.

Referring now to the drawing, the single gure is a circuit diagram, partially schematic, of an aircraft radio transmitter-receiver system ernbodying the invention.

Referring now to the drawing, there is represented a circuit diagram, partially schematic, of an aircraft transmitter-receiver system. This system comprises a voice-signal carrier-frequency selecting, amplifying and detecting rst portion including, in cascade, a radio-frequency amplifier I3, an oscillator-modulator Il, an intermediate-frequency amplier I2 and the diode anodes I3a and cathode I3b of a duo-diode-triode vacuum tube I3. The units I3, Il and I2 may be of conventional construction and operation and are hence illustrated schematically. The input terminals of the radio-frequency ampli- Iier I@ are adapted to be selectively connected to a high-inductance high-Q aircore loop IP., preferably enclosed in an airfoil surface Ilia, or, through a transmitting antenna loading coil Ib, to a retractable trailing antenna I5 having a conical drag Ia. To this end, there is provided amanually operable antenna switch vhaving a movable contact Ia connected to an input terminal of the unit I3 and stationary contacts I 9b, I 9c'connected to the loop I4 and antenna I5, respectively. f

.The units I and II are indicated schematically as including adjustable tuning elements Ia and IIa, respectively, which are connected with an antenna-tuning condenser II by way -of a mechanism indicated schematically at 2| for unicontrol tuning of the apparatus.

`The portion of the receiver including the units IU, II and I2 is preferably designed for operation over the beacon band of ZOO-410 kc. and the broadcast band of 550-1500 kc. To this end, there is provided in the input circuit of the unit I0 a band switch I6 which is connected for unicontrol with band switches Ib and IIb of the units It* and II, respectively, by a mechanism indicated schematically at 22. The band switch I6 is provided with movable bridging contacts 55a and I'- and stationary contacts Ic, Id, Ic and If. The switch I isillustrated in position for operation in the beacon band in which the loop antenna I is connected across the input circuit of the unit Il) through the bridging contact Ita, and the stationary contacts Itd and Ie.

For operation in the broadcast band, the switch I6 is operated to its uppermost dottedline position, in which the input circuit of the unit IU is connected through the bridging contact Ita and the stationary contacts Ic and Itd across the secondary winding of an impedance-matching transformer 23, while the primary winding of this transformer is connected through the bridging contact Io and the stationary contacts Ie and If to Vthe loop anadjustable condenser 24 may be connected across the secondary winding of transformer 23 to tune the loop I4 approximately to the low-frequency end of the broadcast band.

The output circuit of the intermediate-frequency amplifier I2 is connected to the detector comprising the electrodes I3a and |317 and a load'circuitincluding a xed resistor 25 in series with a Volume-control resistor 26 across' which are connected a filter condenser 21. The resistor 26 is provided with a manually adjustable contact 26a which is connected through a coupling condenser 2S to the input electrode or grid I3c of vacuum tube I3 in a conventional manner. The unidirectional component of the signaly across the load circuit 25, 2E, 21 is also applied as an automatic amplication control bias by way of the A. V. C. lead to the control electrodes cf one or more of the tubes of the units I0, II, and I2 in a conventional manner. The anode i3d of the tube I3 is supplied from a suitable source of space current, such as the 10G-volt tap 2da cf a power supply unit 29, through aV load resistor 3B.

So much of the transmitter-received described is substantially conventional in construction and operation so that a detailed explanation of its operation is deemed unnecessary. Briefly, received carrier-frequency voice signals intercepted by the loop antenna I4 or the trailing antenna I5, depending upon the setting of the antenna switch I9, are impressed upon the radio-frequency amplifier Ie in which they are selected and amplified and translated to the oscillatormodulator I I wherein they are converted into an amplified intermediate-frequency carrier signal and from which they are applied to the intermediate-frequency amplifier I2 for further selection and amplication. The intermediate-frequency signal is detected in the circuit including the electrodes i3d and I3b to develop across the load circuit 25, 26 the modulation or voice-signal components. This signal, at an appropriate Volume determined by the setting of the adjustable contact 26a, is amplified by the audio-frequency amplier tube I3 which develops at its output electrode or anode i3d an amplied voice signal for utilization as described hereinafter.

It will be understood that any desired signal in the broadcast or beacon band may be selected by operation of the unicontrol tuning mechanism 2I for adjusting the tuning condenser II and the tuning elements Ia and I Ia of the units I3 and II,.respectively. Selection of signals in the beacon band or broadcast band may be effected by adjustment of the band-switch mechanism 22, which Vis effective to switch from one band selector to another in the units I!! and II and, when switching to operation in the broadcast band, to interpose the impedance-matching transformer 23 between the antenna circuit and the' input circuit of the unit I0 to step down the high impedance of the loop Ui to match the impedance of the input circuit of the unit I0.

The transmitter-receiver system of the invention also includes an auxiliary marker-signal carrier-frequency selecting and detecting portion 3| consisting of a carrier-frequency selector comprising an auto-transformer winding 32 and a tuning condenser 33 proportioned to tune to the marker-signal frequency, presently atV 75 mc; The Winding 32 is provided with an intermediate contact 32a which is connected to an antenna 34, preferably in the form of a quarter-wave-length Whip or rod. Across the tuned circuit 32, 33 is connected a contact-rectier detector consisting ofxa crystal or other contact rectifier 35 in series With a load circuit comprising a resistor 35 and a filter condenser 31 in parallel. 'The detected marker-signal output from the unit 3| is taken out through a shielded or concentric cable 38 and impressed on a movable contact 39a of a manually operable switch 39 having xed contacts 39h and 39e. The switch 39 is illustrated inthe position in which the output signal ofthe markersignal unit 3l is grounded or short-circuited.

The transmitter-receiver system of the invention also includes a carrier-frequency oscillatormodulator and power-amplier portion, or unit I4I), which may be entirely conventional in construction and operation and is therefore shown schematically. The unit 4I) is provided with a radio-frequency output terminal 40a, a B+ and modulation-signal input terminal 40h and a common cathode terminal 43C for the several tubes comprised Within the unit 4D. This unit may be designed to operate at any allocated frequency, for example 3 mc.

The system also includes a plurality of intercommunicating stations, each including a microphone and a signal reproducer, for example a pilot station including a pilot microphone 4I and earphones 42 and one or more passengers stations, each including a microphone 43 and earphones 44.

It will be Seen that the connection from the volume control contact 23a to the input electrode I3c of the audio-amplier tube I3 comprises a circuit for applying to the input electrodes I3c, I 3b of the tube I3 a voice signal translated by the portion of the system including the units I0, II and I2. Across this circuit is connected an impedance element such as a resistor 45, having a value 4of a higher `orderof magnitude than the maximum impedance of the input circuit to the `ampliiier I3 just described. The system also includes a circuit for selectively simultaneously applyngto the input electrodes I3c,` I3b a marker signal translated by the unit 3|. This circuit means includes the switch 39, described above, for selectively applying a marker signal vtranslated by the unit 3| across the impedanceelement'4'5 through 'a shielded or concentric line Miranda coupling condenser 41.

The system of the invention also 'includes a circuit for simultaneously applying to theinput electrodes of the vacuumtube I3 signalsfrom all of the microphones 4I, 43 of the several intercommunicating stations. For. example, ,the miu crophones 4| and 43 are effectively connected in parallel through altransmitting switch, described hereinafter, to a circuit including a low-imped-V ance coupling element connected in series withV the cathode electrode I3b of the amplifier I3. Specifically the microphones 4I and 43 are eilectively connected in parallel across a primary winding 48a of a unity-ratio transformer-48 having a secondary-winding 48h connected between the cathode I3b and ground and provided'with a xed tuning condenser 49 for improving the audio-frequency response of the transformer. Preferably one of the windings of the transformer '48, for example the primary windingita, is loaded with a resistor 590i a lowvalue.

The transmitter-receiver system ofthe invention'also includes-circuit means Vfor coupling all of the signal reproducers, specifically the ear-v phones 42 and 44, to the outputelectrodes:|13b, I3d of the audio-frequency amplier I3. This 'circuit means may be of any suitable type but thereisillustrated by way of example a beampower amplifier 5I having its input electrodes coupled to the anode I-3d of ampliiier I3 through a conventional resistance-capacitance circuit 52, 53. Included in the cathode circuit of the tube 5I is a cathode-bias resistor 54 lay-passed :by a section of a multi-section iilter condenser 55. The unidirectional component of the potential across bias, resistor 54 is applied by way of an isolating resistorz54a and resistor 59 as an operating potential for th-e microphones 4I and 43. Screen and anode potentials are supplied to the tube r5I through a load resistor 58 shunted by a section of the filter condenser 55 and through contacts of the transmitting switch, described hereinafter, from a suitable power output terminal of the unit 29. Included in the output circuit of the beam-power tube 5I is a primary winding 51a of a power-output transformer 51 having a secondary winding 'ib connected directly to the earphones 42 and 44 through a series loading resistor 6E, short-circuitedduring reception by the transmitting switch described hereinafter. The transformer 51 is also provided with a second output winding 51o connected to the radio-frequency input terminal 4Gb of the unit 40.

The system so far described is that of the receiving portion of the system. When so connected the system operates in a conventional manner to translate voice signals from the portion of the system including the units IE), I I and I2 through the amplier I3, the power amplifier 5I, and the output transformer 51 directly to the signal reprcducers 42 and 44. At the same time, by operation of the contact 39a of switch 39 to engage the contact 39e, marker signals derived from unit 3| are impressed across the resistor 45 in the input circuit of the amplifier I3, whereby they are amplified in a manner entirely similar to the voice signals and impressed upon the earphones 42 and 44. Further, with the system connected as described, either the pilot or any of the passengers may talk into his respective microphone 4I, 43 and these intercommunication signals are injected through the low-impedance coupling transformer 48 into the cathode circuit of the amplifier I3 and amplified therein and in the power amplifier 5I in a similar manner and impressed upon all of the earphones 42 and 44. Due to the fact that the resistor 45 is of a higher order of magnitude, preferably three times the highest value of the impedance of the remaining portion of the input circuit of the amplifier I3, and due to the high impedance of the load circuit 35, 31 of the marker-signal unit, this connection of the marker-signal unit 3| to the input electrodes of amplier I3 has no appreciable effect on the operation of the receiver to received voice signals, so that the marker receiver 3| may be connected or disconnected across the resistor 45 without materially affecting the operation of the receiver. Further, due to the extremely low impedance of the coupling `transformer 48 connected in the cathode circuit of the ampliiier I3, the variation of impedance of this circuit reected from the microphones 4I and 43 during operation is negligible in comparison with the anode-cathode impedance of the amplier I3, so that there is an inappreciable coupling between the normal voice-signal translating circuit and the intercommunication circuit. Further, the low impedance of the transformer 48 and the low resistance of its secondary winding 48h avoids substantial degeneration of the audio-frequency signal detected by the diodes |3a. That is, the amplifiers I3 and 5I and the signal reproducers 42 and 44 serve simultaneously for the transmission of voice signals, marker signals and intercommunication signals, without mutual interference, while the system is operating as a receiver.

The transmitter-receiver system of the invention also includes means for modifying the connections of the system described, which constitute a receiving system, to adapt the circuit 'for signal transmission without impairing its performance as a marker-signal receiver or as an intercommunication system. To this end, there is provided a press-to-talk key 58 which is connected to complete the circuit of an operating coil 59a of a transmitting switch or relay 59, the winding 59a being energized from a power-supply terminal 29C of the power-supply unit 29 which may, for example, supply 24 volts D. C. The relay 59 is provided with an armature 59h which, through a link 59C, is adapted to operate two pairs of sliding contacts 59d, 59e and 59j, 59g. The relay 59 is also provided with a series of fixed contacts 59h-59o, inc. The switch 59 is illustrated in its de-energized position in which the several circuits are completed for operation of the system as a receiver, as described above.

The operation of the press-to-talk key 58 to actuate the relay 59 is effective to perform a number of operations simultaneously:

(a) Through the contacts 59k and 59j the common cathode terminal 4c of the unit 49 is grounded, thus completing the energization of the circuits for the several tubes of the unit 49, so that this unit operates in a. conventional manner to develop a carrier-signal output at the terminal 49a which is impressed through the contacts 59m and 59g on the trailing antenna I5, operating as a transmitting antenna. Any communication signals from the pilots microphone 4| will be impressed through the coupling transformer 48 upon the input electrodes of the amplifier I3, amplified therein and impressed upon the power amplifier 5|, in which they are further amplied and applied through the output Winding 51e of transformer 51 upon the modulation input terminal 49h of the unit 46. That is, the switch contacts 59f and 59k effectively couple the output electrodes of the audio-frequency amplier I3 to the transmitting portion 49 of the system by completing the cathodeground connections of the tubes of unit 45.

(b) Simultaneously, the opening of the contacts 59e and 59j opens the circuit to the loading resistor 50 across coupling transformer 49, simultaneously to increase the impedance of this coupling element for increasing the gain of the amplifier I3 to the signals from the pilots mcrophone 4|. This provides a modulation signal of higher amplitude to the input terminal 4Gb of the unit 40, which is desirable for most effective modulation during transmission. On the other hand, during operation of the apparatus as a receiver or as an intercommunicating apparatus, it is desirable to maintain the intercommunication signal at a relatively low level in the output of the amplifiers i3 and 5| in order not to overload the earphones 42, 44 or interfere unduly with the reception of marker signals and this is effected by the low-impedance resistor 5!) in shunt to the coupling transformer 48.

(c) Simultaneously, opening of the circuit at the contacts 59f, E9n removes the short-circuit around resistor 60 and effectively connects this impedance device in series with the secondary winding 51h of output transformer 5l and in series with the signal reproducers 42 and 44 for attenuating the output signals of the amplifier as applied to the earphones 42 and 44. This feature is desirable in order to avoidexcessively loud signals in earphones 42 and 44 from the pilots microphone 4| or from the marker-signal unit 3| due to the increase in gain of the amplifier |3 for communication signals, as described above.

(d) Simultaneously, at the contacts 59e, 59j the circuit is broken for the passengers microphone 43 and for all similar microphones other than that of the pilot so that they are disconnected from the system, permitting only the pilot to-talk during transmitting operation.

(e) Operation of the switch 59 also is effective to connect the screen and anode of the beampower amplifier tube 5| through the contacts 59d and 59h to the high-voltage supply terminal 29h, which may supply, for example 499 volts, still further to increase the gain of the power amplifier tube 5I during transmission.

Thus, there is provided a transmitter-receiver system which while connected for transmitting, still permits the reception of marker signals from unit 3| and still permits reception of voice signals through the normal receiving channels including units I9, and |2 and further permits the pilot to communicate through his microphone 4| with the other intercommunicating stations, such as passenger stations including the earphone 44, the entire changeover from receiving to transmitting being effected by operation of the single press-to-talk key 58.

It is also seen that the amplifiers I3 and.5|, in addition to serving as audio-frequency amplifiers during reception, also serve during transmission as a high-gain modulation amplifier for the pilots microphone 4|, that the sidetone signals translated to the signal reproducers 42v and 44 are attenuated, and that all of the intercommunicating microphones, with the exception of the pilots microphone 4|, are disabled while the system is operating as a transmitter.

While it will be understood that the design constants of the transmitter-receiver system of the invention will vary Within Wide limits in accordance With the particular operating and performance requirements, there follow by Way of example certain of the principal design constants of an amplifier stage I3, 5| in a particular transmitter receiver embodying the invention found to have the desirable operating characteristics described above:

Amplifier I3 Type 12SQ'7 Amplifier 5| Type 6V6 Transformer 48 Unity ratio, 30 ohms resistance, 50 ohms impedance at 400 cycles Resistor 45 3 megohms Resistor 50 20 ohms Resistor 9D 5000 ohms Resistor 54 300 ohms Resistor'54a 1000 ohms l. A radio transmitter-receiver system com-.

prising, a voice-signal carrier-frequency selecting amplifying and detecting iirst portion, an auX- ili'ary carrier-frequency selecting and detecting second portion, a modulator and power amplifier third'portion, at least one intercommunicating microphone, an audio-frequency ampliiier having input means and output means, a circuit for applying to'said input means a voice signal translated by said first portion, a circuit for selectively simultaneously applying to said input means a signal translated by said second portion, a circuit for` simultaneously applying to said input means a signal from said'microphon'e, a signal reproducer coupled to said output means, and means for selectively coupling said 'output means to said third portion, whereby saidampliiir serves as' a modulator amplifier for` said microphone while said system is operating as a transmitter;

2. Aniaircraft radio transmitter-receiverisys'L y tem comprising, a voice-signal carrier-frequency selecting, amplifying and detecting first portion, an auxiliary marker-carrier-frequency selecting and detecting second portion, a carrier-frequency oscillator, modulator and power-amplifier third portion, at least one intercommunicating-microphone, an audio-frequency amplifier having' input means anad output means, a circuit for applying to said input means a voice signal translated by said first portion, a circuit for selectively simultaneously applying to said input means a marker signal translated by said second portion, a circuit for simultaneously applying to said input means a signal from said microphone, a signal reproducer'coupledL to said'output means, whereby said amplifier and reproducer serve simultaneously for the translation of received voice signals, marker-signals and intercommunication-signals, and means for selectively coupling said output means to said third portion, whereby said amplifier serves as a modulation amplifier for said microphone while said system is operating as a transmitter. f

3. An aircraft radio receiver comprising, a voice-signal carrier-frequency selecting, amplifying and detecting first portion, at least one intercommunicating microphone, an audio-frequency amplifier having input, output and cathode electrodes, a circuit for applying to said input electrode a voice signal translated by said first portion, a circuit for simultaneously applying to said cathode a signal from said microphone, and a signal reproducer coupled to said output electrode, whereby said amplifier and reproducer serve simultaneously for the translation of voice signals and intercommunication-signals 4. An aircraft radio receiver comprising, a voice-signal carrier-frequency selecting, amplifying and detecting first portion, at least one intercommunicating microphone, an audio-frequency amplifier having input, output and cathode electrodes, a circuit for applying to said input electrode a voice signal translated by said first portion, a circuit including ay low-impedance coupling element connected in series with said cathode electrode for simultaneously applying to said amplifier a signal from said microphone, and a, signal reproducer coupled to said output electrode, whereby said amplifier and reproducer serve simultaneously for the translation of voice signals and intercommunication-signals.

l 5. An aircraft radio receiver comprising, a voice-signal carrier-frequency selecting, amplifying and detecting first portion, at least one intercommunicating microphone, an audio-frequency amplifier having input, output and cathfit ode electrodes, a circuit for applying to said in-" put electrode a voice signal translated by said first portion, a circuit including a unity-ratiolowimpedance transformer connected in series-with said cathode electrode for simultaneously applying to said amplifier a signal from said microphone, and a signal reproducer coupled to said output electrode, whereby said amplifier and reproducer serve simultaneously for the'ti'anslation of voice signals and intercommunication-signalsL 6. An aircraft radio transmitter-receiver system comprising, a voice-signal carrier-frequency selecting, amplifying and detecting first portion, a carrier-frequency oscillator, modulator and power amplifier second portion, at least onev intercommunicating microphone, an audio-frequency ampliiier having input means and output means; a circuit for applying to said input means a'voice signal translated by said first portion, a circuit for simultaneously applying to said inputl means a signal from said microphone, a signal reproducer coupled to said output means, whereby-said amplifier and reproducer serve simultaneously for the translation of voice signals and intercommunication-signals, and means for selectively coupling said output means to said second'portion and for simultaneously increasing the gain of said amplifier to microphone signals, whereby said amplifier serves as a modulation amplifier for said microphone while said system is operating as a transmitter.

'7. An aircraft radio transmitter-receiver system comprising, a voice-signal carrier-frequency selecting, amplifying and detecting first portion, a carrier-frequency oscillator, modulator and power amplifier second portion, at least one intercommunicating microphone, an audio-frequency amplifier having input means and output means, a circuit for applying to said input means a voice signal translated by said iirst portion, a circuit for simultaneously applying to said input means a signal from said microphone, a signal reproducer coupled to said output means, whereby said amplifier and reproducer serve simultaneously for the translation of voice signals and intercommunication-signals, and means for selectively coupling said output means to said second portion and for simultaneously increasing the gain of said amplifier to microphone signals and attenuating the output signals thereof as applied to said reproducer, whereby said amplifier serves as a modulation amplifier for said microphone while said system is operating as a transmitter.

8. An aircraft radio transmitter-receiver system comprising, a voice-signal carrier-frequency selecting, amplifying and detecting first portion, a carrier-frequency oscillator, modulator and power amplifier second portion, at least one intercommunicating microphone, an audio-frequency amplifier having input, output and cathode electrodes, a circuit for applying to said input electrode a voice signal translated by said iirst portion, a circuit including a low-impedance coupling element connected in series with said cathode electrode forlsimultaneously applying to said amplier a signal from said microphone, a signal reproducer coupled to said output electrodes, whereby said amplifier and reproducer serve simultaneously for the translation of voice signals and intercommunication-signals, and means for selectively coupling said output electrode to said second portion and for simultaneously increasing the impedance of said coupling element and connecting an impedance device in series with 11 said sound reproducer, whereby said amplifier serves as a modulation amplifier for said microphone while said system is operating as a transmitter.

9. An aircraft radio transmitter-receiver system comprising, a voice-signal carrier-frequency selecting, amplifying and detecting first. portion, a carrier-frequency oscillator, modulator and power amplifier second portion, a plurality of intercommunicating stations each including a microphone and a signal reproducer, an audiofrequency amplifier having input means and output means, a circuit for applying to said input means a voice signal translated by said first portion, a circuit for simultaneously applying to said input means signals from said microphones, a signal reproducer coupled to said output means, whereby said amplifier and reproducer serve simultaneously for the translation of voice signals and intercommunication-signals, and means for selectively coupling said outputl means to said second portion and for disconnecting all but one of said microphones from said system, whereby said amplifier serves as a modulation amplifier for said microphone and said intercommunicating stations are disabled while the systemis oprating as a transmitter.

'10. A radio receiver comprising a voice-signal carrier-frequency selecting, amplifying and detecting rst portion, an ntercommunicating microphone, an audio-frequency amplier having a tube with grid, anode and cathode electrodes, a circuit for applying to said grid electrode a voice signal translated by said first portion, a circuit including a low-impedance coupling element connected to said cathode electrode, means connected to said microphone and inductively coupled to said low-impedance coupling element for applying to said amplifier a 5 signal from said microphone, the impedance of said coupling element at audio frequencies being negligible in comparison with the anode-cathode impedance of said tube and a signal reproducer coupled to said output electrode, whereby said amplifier and reproducer serve simultaneously for the translation of voice signals and intercommunication signals.

DONALD H. MITCHELL.

15 REFERENCES CITED UNITED STATES PATENTS Sziklai Dec. 7, 1948 OTHER REFERENCES y Principles of Aeronautical Radio Engineering by Sandretto published by McGraw-Hill Co., New 35 York, N. Y., 1942*, pages 366-379; 

